|
Return to
Failure Analysis Case Histories
|
|
Under-Coating Corrosion of Tin Coated
Copper |
|
EQUIPMENT:
|
Radiator Fins |
|
ENVIRONMENT:
|
De-Ionized Water Spray |
|
MATERIAL: |
Tin Coated Copper |
|
SERVICE TIME:
|
2 to 3 years |
|
FAILURE:
|
Under-Coating Corrosion |
|
|
|
The radiator fins are joined to copper tubing with 100% tin solder by dipping
the pre-assembled parts in a molten tin bath. The radiator assemblies are
fundamental parts of humidifiers installed in the paint rooms of an automotive
assembly plant. The humidifying effect is achieved by spraying de-ionized water
over the heated copper tubing. Excess water is collected and recirculated.
|
 |
| Figure 1. Pit initiation under coating (1000X
Original Magnification) |
|
Visual examination of the fins revealed areas of varying degrees of corrosive
attack. Areas containing very light attack with small pin holes through the tin
coating to heavily attacked areas where the copper substrate was exposed and
through penetration pitting was observed. |
|
SEM (scanning electron microscope) equipped with EDS (energy dispersive x-ray
spectroscopy was used to identify the elemental components of the corrosion
products. Green deposits observed on the surface were found to contain mainly
tin, copper and chlorine. Green and black corrosion products from inside the
pits were found to contain mainly copper, chlorine and sulfur. The chloride
content of the corrosion products found in the pits was noticeably higher than
that found on the surface. |
|
|
|
A cross-section revealed substantial under-coating corrosion (Figure 1). In
this type of corrosion, pits develop in the substrate material at imperfections in the coating. As the pit enlarges, the substrate no longer
provides support for the coating. Eventually, the coating will either spall
off or cave in. The progression of "cave-in" under coating corrosion is
illustrated in Figures 2 and 3.
|
|
This failure was most likely caused as a result of the water supply becoming
contaminated with sulfur and chloride compounds.
|
